$\text D = \left[\begin{array}{rr}4 & -1 \\ 2 & -1\end{array}\right]$ What is the determinant of $\text D$ ?
The determinant of a 2x2 matrix can be computed the following way: $ \left|\begin{array}{rr}a & b \\ c & d\end{array}\right| = (a \times d)-(b \times c) $ In this specific case, $ \left|\begin{array}{rr}4 & -1 \\ 2 & -1\end{array}\right| = (4 \cdot -1)-(-1 \cdot 2) $ $ = -2 $